This invention relates to lightning detection and data acquisition systems, and in particular to systems that provide continuous lightning detection and are programmable to allow for user-selectable evaluation criteria.
Lightning detection and data acquisition systems are used to detect the occurrence and determine the location of lightning discharges, and gather other data about the discharges. In traditional lightning detection systems, a plurality of sensors are placed tens to hundreds of kilometers apart to remotely detect the electric and magnetic fields of lightning discharges. Such discharges may be between a cloud and the ground (xe2x80x9cCGxe2x80x9d) or within a cloud (xe2x80x9cICxe2x80x9d). Information from the sensors is transmitted to a central location, where analysis of the sensor data is performed. Typically, at least the time of occurrence and location of the discharges are determined from data provided by a plurality of sensors.
Remote sensors of lightning detection and data acquisition systems typically detect electric and magnetic fields of both CG and IC lightning flashes, which are composed of many discharges. It is often important to be able to distinguish between the two types of flashes. To that end, remote sensors often look at the low-frequency (xe2x80x9cLFxe2x80x9d) and very-low-frequency (xe2x80x9cVLFxe2x80x9d) emissions from lightning discharges. The electrical signals produced by LF and VLF (xe2x80x9cLF/VLFxe2x80x9d) detectors are ordinarily integrated prior to analysis to produce a waveform representation of the electric or magnetic discharge field, as the antenna inherently responds to the time derivative of the field. Analyzing signals representative of either an electric or magnetic field to distinguish CG and IC discharges is referred to as performing waveform analysis. There are several criteria for distinguishing between CG and IC events. One well known method for distinguishing lighting signals both in the LF and in the VLF range is to examine the time that passes from a peak in a representative signal to the instant it crosses a zero amplitude reference point. This is referred to as a peak-to-zero (xe2x80x9cPTZxe2x80x9d) method of analysis. A relatively short PTZ time is a good indication that an IC discharge has occurred. Another well known method of distinguishment is referred to as a bipolar test wherein the representative signal is examind for a first peak and a subsequent peak of opposite polar which is greater than a predetermined fraction of the first peaks. Such an occurrence is another good indication of an IC discharge. Yet another test for IC discharges is the presence of subsequent peaks of the same polarity in a representative signal greater than the initial peak. This is predicated on the fact that some IC discharges have a number of small and fast leading electromagnetic pulses prior to a subsequent larger and slower pulse. In the absence of such criteria indicating that the discharge is an IC discharge, it is ordinarily assumed to be a CG discharge. Even with the application of all established criterion for distinguishing between CG and IC events, some events are still misclassified.
An alternative method of lightning detection is to monitor very high frequency (xe2x80x9cVHFxe2x80x9d) radiation from lightning discharges. However, VHF detection systems must be able to process information at extremely high data rates, as VHF pulse emissions in IC lightning occur approximately one tenth of a millisecond apart. Additionally, VHF systems can only detect lightning events that have direct line of sight to the sensor. One such system is currently in use by NASA at Kennedy Space Center in Florida However, this system is further restricted to line of sight between the sensors and the central analyzer as it uses a real-time microwave communication system. Additionally, the VHF system in use by NASA has proven to be expensive to install and maintain.
Previous lightning detection and data acquisition systems for detecting low frequency electric field signals have been designed around a combination of two location methods, time-of-arrival (xe2x80x9cTOAxe2x80x9d) and magnetic direction finding, with time-domain field waveform analysis. In most of these systems, the sensors are predominately analog devices. Using analog devices in lightning sensors requires the utilization of xe2x80x9ctrack and holdxe2x80x9d circuits to detect a quailing event, capture a representative signal, and perform waveform analysis on it. Due to an accumulation of delay periods in these xe2x80x9ctrack and holdxe2x80x9d circuits, these sensors have a large xe2x80x9cre-armxe2x80x9d time, or xe2x80x9cdead-timexe2x80x9d, during which the sensors do not record subsequent lightning events. Even more modern lightning detection and data acquisition systems that are substantially digital have some dead time. For example, the sensors in some such systems have a xe2x80x9cdead-timexe2x80x9d of 5 to 10 milliseconds, and even the most current digital sensors have a xe2x80x9cdead-timexe2x80x9d of up to one millisecond. The latter are capable of detecting only a limited fraction of IC lightning discharges. This is due in part to the fact that several IC lightning discharges could occur in a single millisecond. CG lightning flashes, however tend to have fewer discharges with relatively large periods of times between individual discharges. If a previous generation sensor is designed to monitor both CG and IC electric field signals, a significant portion of time is occupied processing IC discharge events at the expense of recording CG events. Another aspect associated with sensor dead times and the TOA location method is the uncertainty in assuring that multiple remote sensors will respond to the same IC lightning event. Due to attenuation suffered by electromagnetic waves as they travel long distances over the earth, remote small amplitude events become difficult to detect. If different sensors produce time-of-arrival information from different events, the computed discharge location will have significant error.
Analog sensors operating at LF/VLF frequencies are difficult to tune for both CG and IC lightning discharges. The median amplitude of a CG field signal is about an order of magnitude greater than the median amplitude of an IC field signal. Optimizing the gain of one of these sensors to detect IC events often causes the sensor to become saturated with the much greater energy of nearby CG lightning discharges. Therefore, it is customary to adjust the gain to accommodate both types of field signals, reducing a sensor""s ability to detect IC events. As distant IC lightning discharges become attenuated by propagation over the ground, they become difficult to distinguish from background environmental noise.
In order for the lightning detection system to provide useful information in a timely manner, there must exist a method of transmitting sensor information to a central location. This central location must collect information from numerous remote sensors which is then correlated to establish the location, magnitude, and time of occurrence of lightning discharges. Existing detection systems generally have low-bandwidth communication systems, limiting the amount of information that a sensor can transmit to the central analyzer. In many existing lightning detection networks, the sensors are connected to a central location by low-speed telephone modems, usually 2400 to 9600 bits per second. In the past, this communication restriction was not overly critical, as the large dead-time of previous generation analog sensors limited the amount of information that could be collected and sent to the central analyzer.
Once the sensor information arrives at a central location, it must be analyzed. The information from each sensor is compared against incoming information from other sensors. This correlation process attempts to find corresponding data to determine the location, magnitude, and time of occurrence of lightning discharges. However, current correlation techniques are not sufficient to handle large amounts of information when the time between discharges is more than an order of magnitude shorter than the travel time between sensors. In fact, if a lightning detection system made use of advanced technologies to transmit and receive an increased amount of information, current central analyzers would be unable to process the information efficiently with current correlation techniques.
The state of the art of lightning detection and data acquisition systems is generally represented, in part, by several patents. First, Krider et al. U.S. Pat. Nos. 4,198,599 and 4,245,190 describe a network of gated wideband magnetic direction finding sensors. These sensors are sensitive to return strokes in CG lightning flashes. In U.S. Pat. No. 4,198,599, discrimination and classification is accomplished by examining the shape of the time-domain field waveform. A short rise time (time from threshold to peak) results in a representative signal being placed in an analog track and hold circuit while further analysis is performed. These sensors are designed with CG discharges being of primary interest. Any IC lightning discharges that are detected are discarded. However, both CG and IC events that meet the short rise time criteria and a simple test of event duration result in a significant amount of sensor dead-time.
Second, Bent et al. U.S. Pat. Nos. 4,543,580 and 4,792,806 disclose networks of sensors that measure TOA of electric field signals and employ this information to locate lightning. These sensors do not discriminate between IC and CG discharges. However, these sensors suffer the similar dead time issue as the magnetic direction sensors of the Krider patents. When a number of IC discharge pulses occur in a short time, there is no assurance that multiple sensors will respond to same IC discharge event.
Another patent of interest is Markson et al. U.S. Pat. No. 6,246,367 wherein a lightning detection system utilizes an analog-to-digital converter (xe2x80x9cADCxe2x80x9d) to provide continuous processing of representative field signals. This eliminates the dead time issue inherent in previous generation sensors. Markson describes using a bipolar comparator to distinguish between positive and negative polarity versions of a particular pulse that is inferred to be the first broadband radiation pulse in either a CG or an IC flash. Markson also uses a data correlation process and time-of-arrival difference location method. Markson explicitly uses a high pass filter to block most low frequency components of representative field signals, which are not necessarily useful for detecting the initial pulse in the flash. Limitations of the Markson patent are the specific use of the HF frequency range and detection and processing of only the first pulse in each flash.
Accordingly, there has been a need for improvement of lightning detection and data acquisition systems in several respects. First, an improved signal conditioning method is needed. CG events are normally an order of magnitude larger than IC events at LF, due to the channel length and amount of current which flows during a CG return stroke. As mentioned previously, increasing the gain, or equivalently reducing the event threshold, results in CG events saturating an analog detection and evaluation system or the pick-up of significant amounts of noise. Reducing the gain, or equivalently increasing the event threshold, results in inefficient detection that masks IC events. There is a need to reduce the effect of this magnitude difference between CG and IC signals while removing unwanted noise components. An interesting aspect of both electric field and magnetic field antennae is that they produce a signal which is proportional to the time derivative of the electromagnetic field they are detecting. These differentiating antennae actually reduce the magnitude disparity between IC and CG differential representative signals. However, current generation sensors invariably impose integration methods to convert the differentiated field signal to one representative of the electromagnetic field without making use of the fact that the antenna itself reduces dynamic range requirements. Additionally, there is a need for an improved classification method for distinguishing between lightning types.
Another need in the industry is the ability to program remote sensors with new or different waveform analysis techniques. There is also a need for improved data compression and data decimation techniques to accommodate more IC as well as CG information. Additionally, new data correlation techniques are needed to handle increased information processing rates. These correlation techniques need to handle both time-of-arrival and direction information.
Thus, a need exists for a complete lightning detection and data acquisition system that combines new methods of signal conditioning, a user changeable system for event classification, new methods of data compression, and new data correlation techniques to efficiently detect CG and IC events and determine their location, magnitude, and time of occurrence.
The present invention meets the aforementioned needs by utilizing a plurality of remote programmable sensors (RPS) disposed in different geographic locations to detect, classify, package, and transmit in compressed form information regarding both CG and IC lightning discharges. The information is collected at a central analyzer location where it is decompressed and correlated in order to determine the location, magnitude, and time of occurrence of the lightning discharges. An antenna designed to detect the electromagnetic field signal from a lightning discharge and produce a derivative representative field signal is used. The derivative signal has the benefit of reducing the amplitude disparity between CG and IC field signals. A filter is used to increase the signal to noise ratio by passing the low frequency portions of the differentiated signal while discarding high frequency noise without integrating the principal components of the signal. Non-linear amplification further reduces the amplitude disparity between CG and IC signals by providing greater amplification for lower amplitude signals. The amplified signals are then processed by an ADC to convert an amplified differential signal into a digital representation. This conversion allows a signal to be processed and stored digitally. The digital representation is then integrated by a digital processor to provide a signal representative of the electric or magnetic field. The digital differentiated field signal and the digital signal representative of the field itself are used by the digital processor to classify the lightning event as either a CG or IC event. The analog-to-digital conversion coupled with digital storage permits continuous detection and evaluation of lightning discharges, which eliminates the xe2x80x9cdead timexe2x80x9d inherent in previous generation lightning detection systems.
The present invention uses a novel data compression process to transmit data over low bandwidth communication channels. Numerous digital signal pulses representative of lightning discharges are grouped together in pulse trains. The largest pulse is designated as the reference pulse and its amplitude, time, and direction (if available) are included in a data record. Other pulses in the pulse train are represented by a fractional amplitude of the reference pulse and a time-stamp relative to the time of the preceding or following pulse. This greatly reduces the information that must be transmitted to define all the pulses in the pulse train accurately. If the amount of transmitted information still exceeds the bandwidth of an associated communications channel then the RPS sensors in the lightning detection system can be programmed to transmit synchronized portions of the information, so that all sensors will report information about the same lightning events.
Once received by the central analyzer, the information is unpacked and the original pulse amplitude, time, and direction (if available) information is reconstructed. The unpacked pulse information is used to correlate lightning strike information from a plurality of sensor locations. This information is used to determine the magnitude, location, and time of occurrence of the lightning discharge.
Accordingly, it is a principal object of the present invention to provide a novel and improved lightning detection and data acquisition system and method.
It is another object of the present invention to provide a lightning detection and data acquisition system and method with improved capability of distinguishing CG and IC lightning events.
It is a further object of the present invention to provide a lightning detection and data acquisition system that reduces the amplitude disparity between CG and IC lightning representative field signals.
It is an additional object of the present invention to provide a lightning detection and data acquisition system and method that provides continuous detection and processing of electromagnetic field signals caused by lightning discharges.
It is yet another object of the present invention to provide a lightning detection and data acquisition system and method for compression, decimation, and transmission of digital representations of lightning electromagnetic field signals.
It is yet a further object of the present invention to provide a lightning detection and data acquisition system and method for improved correlation of information from a plurality of remote programmable sensors to determine the location, magnitude, and time of occurrence of lightning strikes.
It is a further object of the invention to provide a lightning detection and data acquisition system in which the configuration of the sensors may be set or altered by remote access.
The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.